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Cardiac Arrest (CA) is essentially a loss of effective cardiac activity. CA typically occurs as the result of a nonperfusing cardiac arrhythmia. The most common nonperfusing cardiac arrhythmias are ventricular fibrillation, pulseless ventricular tachycardia, pulseless electrical activity, asystole and pulseless bradycardia.
Typically, in the absence of a verifiable do-not-resuscitate order, CPR should be performed on any subject who becomes unconscious and is found to be pulseless. CPR typically consists of the administration of cardiac compressions in combination with artificial ventilation to maintain at least MEBF and oxygenation during cardiac arrest. The cardiac compressions may be administered manually and/or by way of a cardiac compression device such as the AutoPulse® Noninvasive Cardiac Support Pump (ZOLL Circulation, Sunnyvalle, Calif.).
CPR cardiac compressions are typically initiated as soon as possible after the subject becomes pulseless and are continued until either ROSC has been established or death is pronounced. In some cases, ROSC may occur before spontaneous or fully effective respiration has occurred. Thus, it is not uncommon for a subject to remain intubated and on ventilation support even after ROSC has occurred and cardiac compressions have been stopped.
Some CA subjects in whom ROSC is successfully restored may subsequently undergo repeat CA, thereby necessitating recommencement of cardiac compressions and further continuance of CPR until either ROSC has once again occurred or death is pronounced.
The ability to determine precisely when CA or ROSC has occurred is desirable to enable rescuers treating the subject to know when to start or stop administering cardiac compressions. Additionally, while CPR is ongoing, it is desirable have some feedback as to how much endovascular blood flow is actually being created by the cardiac compressions so that rescuers may know whether the cardiac compressions are being administered with sufficient force and in a manner that is creating at least MEBF.
In clinical settings where a CA subject is intubated and on ventilation support, a respiratory monitoring technique known as quantitative waveform capnography has been used to determine the occurrence of ROSC as well as to assess whether cardiac compressions are being performed in a manner that attains at least MEBF. Typically, quantitative waveform capnography involves the use of a device known as a capnograph to continuously measure a ventilation parameter known as end-tidal carbon dioxide or ETCO2 (sometimes alternatively referred to as PetCO2). ETCO2 is a direct measurement of ventilation in the lungs. It is also useable as an indirect indicator of blood circulation (i.e., a decrease in circulatory perfusion decreases the rate at which carbon dioxide is exhaled through the lungs thereby decreasing ETCO2. ETCO2 in an adult patient with normal spontaneous circulation is around 35-45 mmHg. In an intubated CA subject who is undergoing CPR, an ETCO2 of less than 10 mmHg may be an indication that MEBF has not achieved and that the manner in which the cardiac compressions are being applied may require some modification.
When effective cardiac compressions are given during CA, the ETCO2 value is expected to be 10-20 mmHg. When ROSC occurs, the ETCO2 then increases to 35-45 mmHg. Thus, in an intubated patient cardiac arrest patient, quantitative waveform capnography has been suggested as a tool for monitoring the effectiveness of CPR cardiac compressions and determining when ROSC occurs. However, in order to perform quantitative waveform capnography the patient typically must be intubated and connected to a capnography device that is programmed to monitor ETCO2.
Today, many critically ill patients are treated with Endovascular Temperature Management (ETM). In ETM, a heat exchange catheter is inserted into the patient's vasculature and connected to a console which generally includes a programmable controller, heater, cooler, pumping apparatus and user interface. A desired target temperature may be input, via the user interface, and the controller will then cause heated or cooled heat exchange fluid to be circulated through the heat exchange catheter. The exchange of heat between the circulated heat exchange fluid and the patient's blood flowing past the heat exchange catheter causes the patient's core body temperature to be raised or lowered to the target temperature. The ETM system will then maintain the patient's body temperature at or near the target temperature until the patient is returned to normothermia and the ETM treatment is discontinued. Such ETM systems are currently available from ZOLL Circulation, Inc., Sunnyvale, Calif. and Phillips-Innercool, San Diego, Calif.
In addition to being useable for controlling the patient's body temperature, ETM systems can also be programmed to compute the patient's blood flow rate based on the rate at which heat is exchanged between the heat exchange fluid and the patient's blood. Essentially, the faster the blood flow rate the greater the rate of heat exchange and vice versa. Details as to the manner in which ETM systems may be used to determine blood flow rate are described in U.S. Pat. No. 7,087,026 (Callister, et al.) entitled Devices and Methods for Measuring Blood Flow Rate or Cardiac Output and for Heating or Cooling the Body, the entire disclosure of which is expressly incorporated herein by reference.
It would be advantageous if heat exchange catheters and associated control equipment used for ETM could be additionally modified to provide real time indications of noteworthy blood-flow events such as the occurrence of cardiac or circulatory arrest and/or whether cardiac compressions being administered during CPR are effective to create MEBF and/or when ROSC has occurred.